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David Evans CS201j: Engineering Software University of Virginia Computer Science Lecture 23: Everything Else You Should.

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Presentation on theme: "David Evans CS201j: Engineering Software University of Virginia Computer Science Lecture 23: Everything Else You Should."— Presentation transcript:

1 David Evans CS201j: Engineering Software University of Virginia Computer Science Lecture 23: Everything Else You Should Know (but wont see on Exam 2)

2 20 November 2003CS 201J Fall Menu Course Evaluations 101 Things Every Computer Scientist should know Course Charge

3 20 November 2003CS 201J Fall Course Pledge You Signed I will provide useful feedback. I realize this is an experimental course and it is important that I let the course staff know what they need to improve the course. I will not wait until the end of the course to make the course staff aware of any problems. I will provide feedback either anonymously (using the course feedback form) or by contacting the course staff directly. I will fill out all course evaluation surveys honestly and thoroughly.

4 20 November 2003CS 201J Fall Two Surveys Required Official SEAS Survey –You receive about it –Administrators read it to determine whether or not to fire me CS201J Specific Survey: handed out today –Specific questions to help improve future editions of CS201 I do read all of the surveys completely Department head and curriculum committee will read SEAS survey

5 20 November 2003CS 201J Fall Questions 000What is Computer Science? 001What problem did the first electronic programmable computer solve? 010What are the worlds most complex programs? 011How do Computer Scientists manage complexity? 100 What is and is not computable? 101 Who invented the Internet?

6 20 November 2003CS 201J Fall What is Computer Science?

7 20 November 2003CS 201J Fall Let AB and CD be the two given numbers not relatively prime. It is required to find the greatest common measure of AB and CD. If now CD measures AB, since it also measures itself, then CD is a common measure of CD and AB. And it is manifest that it is also the greatest, for no greater number than CD measures CD. Euclids Elements, Book VII, Proposition 2 (300BC)

8 20 November 2003CS 201J Fall The note on the inflected line is only difficult to you, because it is so easy. There is in fact nothing in it, but you think there must be some grand mystery hidden under that word inflected! Whenever from any point without a given line, you draw a long to any point in the given line, you have inflected a line upon a given line. Ada Byron (age 19), letter to Annabella Acheson (explaining Euclid), 1834

9 20 November 2003CS 201J Fall What is the difference between Euclid and Ada? It depends on what your definition of is is. Bill Gates (at Microsofts anti- trust trial)

10 20 November 2003CS 201J Fall Geometry vs. Computer Science Geometry (mathematics) is about declarative knowledge: what is Computer Science is about imperative knowledge: how to –Ways of describing imperative processes (computations) –Ways of reasoning about (predicting) what imperative processes will do Language Logic

11 20 November 2003CS 201J Fall Was CS201J a Computer Science Course? No! –Only about 30% of CS201J is computer science. –Most of it is software engineering. If you want to take a Computer Science course, Consider taking CS200 in the Spring

12 20 November 2003CS 201J Fall What problem did the first electronic programmable computer solve?

13 20 November 2003CS 201J Fall Colossus First Programmable Computer Bletchley Park, 1943 Designed by Tommy Flowers 10 Colossi in operation at end of WWII Destroyed in 1960, kept secret until 1970s 2 years before ENIAC – calculating artillery tables

14 20 November 2003CS 201J Fall Colossus Problem Decode Nazi high command messages from Lorenz Machine XOR encoding: C i = M i K i –Perfect cipher, if K is random and secret

15 20 November 2003CS 201J Fall For any given ciphertext, all plaintexts are equally possible. Ciphertext: Key: Plaintext: = CS Why perfectly secure? 1 0 B

16 20 November 2003CS 201J Fall Breaking Lorenz Operator and receiver need same keys Generate key bits using rotor machine, start with same configuration One operator retransmitted a message (but abbreviated message header the second time!) Enough for Bletchley Park to figure out key – and structure of machine that generated it! But still had to try all configurations

17 20 November 2003CS 201J Fall Colossus Read ciphertext and Lorenz wheel patterns from tapes Tried each alignment, calculated correlation with German Decoded messages (63M letters by 10 Colossus machines) that enabled Allies to know German troop locations to plan D-Day

18 20 November 2003CS 201J Fall What are the worlds most complex programs?

19 20 November 2003CS 201J Fall Complex Programs Apollo Guidance Software –~36K instructions F-22 Steath Fighter Avionics Software –1.5M lines of code (Ada) 5EEE (phone switching software) –18M lines Windows XP –~50M lines (1 error per kloc ~ 50,000 bugs) Anything more complex?

20 20 November 2003CS 201J Fall Human Genome Produces 60 Trillion Cells (6 * ) 50 Million die every second!

21 20 November 2003CS 201J Fall DNA Sequence of nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T) Two strands, A must attach to T and G must attach to C A G T C

22 20 November 2003CS 201J Fall Codons Three nucleotides encode an amino acid Sequence of amino acids makes a protein But, there are only 20 amino acids, so there may be several different ways to encode the same one From

23 20 November 2003CS 201J Fall Central Dogma of Biology (Crick, 1957) DNA makes RNA RNA makes proteins Proteins make us DNA Transcription RNA Translation Protein Image from

24 20 November 2003CS 201J Fall Shortest (Known) Life Program Nanoarchaeum equitans –490,885 bases (522 genes) = 490,885 * ¼ * 21/64 = 40,268 bytes –Parasite: no metabolic capacity, must steal from host –Complete components for information processing: transcription, replication, enzymes for DNA repair Size of compiling C++ Hello World: Windows (bcc32): 112,640 bytes Linux (g++): 11,358 bytes KO Stetter and Dr Rachel Reinhard

25 20 November 2003CS 201J Fall The Make-Human Program 3 Billion Base Pairs –Each nucleotide is 2 bits (4 possibilities) –3B bases * 1 byte/4 pairs = 750 MB –Highly redundant encoding (21/64) ~ 250 MB Only 5% is transcribed (exons) ~ 12 MB –95% junk (intons): genomes from viruses reverse transcribed into human genome, but inactive Wal-Marts database is 280 Terabytes CD = 650 MB

26 20 November 2003CS 201J Fall Expressiveness of DNA Genetic sequence for 2 humans differs in only 2 million bases –4 million bits = 0.5 MB 1/3 of a floppy disk <1% of Windows 2000

27 20 November 2003CS 201J Fall How do Computer Scientists manage complexity?

28 20 November 2003CS 201J Fall Abstraction Adapted from Gerard Holzmanns FSE Slides

29 20 November 2003CS 201J Fall Abstraction in CS201J Abstraction by Specification –Abstract away how details by saying what a procedure does Data Abstraction –Abstract away representation details by specifying what you can do with something Subtyping –Abstract away actual type details by allowing many types to be used in the same way (?) Concurrency –Abstract away when details (?)

30 20 November 2003CS 201J Fall What is and is not computable?

31 20 November 2003CS 201J Fall Halting Problem Input: a procedure P Output: true if P halts (finishes execution), false otherwise. Is it possible it implement a procedure that correctly implements halts and always terminates?

32 20 November 2003CS 201J Fall Halts is not Computable boolean contradictHalts (Program P) if (halts contradictHalts (P);) while (true) ; else return true; If contradictHalts halts, the if test is true if enters the while loop - it doesnt halt! If contradictHalts doesnt halt, the if test if false, and it evaluates to true. It halts!

33 Learned Discussion on Computability (Video)

34 20 November 2003CS 201J Fall Ali G Multiplication Problem Input: a list of n numbers Output: the product of all the numbers Is it computable? Yes – a straightforward algorithm solves it. Can real computers solve it?

35

36 20 November 2003CS 201J Fall Ali G was Right! Theory assumes ideal computers: –Unlimited memory –Unlimited power –Unlimited (finite) time Real computers have: –Limited memory, time, power outages, flaky programming languages, etc. –There are many decidable problems we cannot solve with real computer: the numbers do matter

37 20 November 2003CS 201J Fall Who Invented the Internet? skip

38 20 November 2003CS 201J Fall What is a Network? A group of three or more connected entities communicating indirectly Ancient Greeks had beacon chain networks on Greek island mountaintops

39 20 November 2003CS 201J Fall Chappes Semaphore Network Mobile Semaphore Telegraph Used in the Crimean War First Line (Paris to Lille), 1794

40 20 November 2003CS 201J Fall internetwork A collection of multiple networks connected together, so messages can be transmitted between nodes on different networks.

41 20 November 2003CS 201J Fall The First Internetwork 1800: Sweden and Denmark worried about Britain invading Edelcrantz proposes link across strait separating Sweden and Denmark to connect their (signaling) telegraph networks 1801: British attack Copenhagen, transmit message to Sweden, but they dont help. Denmark signs treaty with Britain, and stops communications with Sweden

42 20 November 2003CS 201J Fall First Use of The Internet October 1969: First packets on the ARPANet from UCLA to Stanford. Starts to send "LOGIN", but it crashes on the G. 20 July 1969: Live video (b/w) and audio transmitted from moon to Earth, and to several hundred million televisions worldwide.

43 20 November 2003CS 201J Fall Licklider and Taylors Vision Available within the network will be functions and services to which you subscribe on a regular basis and others that you call for when you need them. In the former group will be investment guidance, tax counseling, selective dissemination of information in your field of specialization, announcement of cultural, sport, and entertainment events that fit your interests, etc. In the latter group will be dictionaries, encyclopedias, indexes, catalogues, editing programs, teaching programs, testing programs, programming systems, data bases, and – most important – communication, display, and modeling programs. All these will be – at some late date in the history of networking - systematized and coherent; you will be able to get along in one basic language up to the point at which you choose a specialized language for its power or terseness. J. C. R. Licklider and Robert W. Taylor, The Computer as a Communication Device, April 1968

44 20 November 2003CS 201J Fall The Modern Internet Packet Switching: Leonard Kleinrock (UCLA), Donald Davies and Paul Baran, Edelcrantzs signaling network (1809) Internet Protocol: Vint Cerf, Bob Kahn Vision, Funding: J.C.R. Licklider, Bob Taylor Government: Al Gore (first politician to promote Internet, 1986; act to connect government networks to form Interagency Network)

45 20 November 2003CS 201J Fall Charge 1 Exam 2 –Out today –Due Friday at 3:55PM –Turn in to folder outside my office

46 20 November 2003CS 201J Fall Charge 2 Course Evaluations –SEAS Survey: follow the instructions –Course Specific survey Handed out today Turn in to folder outside my office when you turn in your exam

47 20 November 2003CS 201J Fall Charge 3 Change the World! Caveat: before worrying about changing the world, make sure you turn in your exam Friday!

48 20 November 2003CS 201J Fall Small, Simple Programs that Changed the World VisiCalc (Dan Bricklin and Bob Frankston) WorldWideWeb (Tim Berners-Lee) Tetris (Alexey Pazhitnov) Napster (Shawn Fanning and Sean Parker) eBay (P. Omidyar) ? (CS201J Student) Altair BASIC (Bill Gates and Paul Allen) Smalltalk (Adele Goldberg, Alan Kay, Dan Ignalls)

49 Lawn Lighting: 7pm Thursday Never doubt that a small group of thoughtful, committed people can change the world. Indeed, it is the only thing that ever has. Margaret Mead


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